A pay television descrambling system is composed of a receiver and a security module. This module may be detachable or fixed. The purpose of the receiver is to descramble the signals received. The main purpose of the security module is to control the operation by verifying the descrambling authorization and by providing, as required, the information necessary for operating the descrambling module, for example by providing descrambling vectors, also called control words.
The existing systems use receivers which interrogate their security. modules, and the latter reply by providing the information necessary for descrambling. At a given time, for the same transmitted broadcast, all the security modules reply with the same information. Because the information flows between the security module and the receiver at a low rate (of about 20 to 30 bytes/s), defrauders may use this information in order to broadcast it publicly, for example via the Internet.
Such a system is described in the document “DVD Conditional Access” written by David Cutts in the journal called “Electronics & Communication Engineering Journal” of February 1997. It is described a standardized system to unscramble audio and video streams coming from different service providers. The security module, called CA (Conditional Access) is in charge of delivering the control words CW. This module can additionally uses a smart card (SM) for the descrambling function. Once the information are descrambled in the CA module, the control words CW are returned in the decoder for obtaining the uncoded information.
In a prior publication entitled “Conditional Access Broadcasting:Datacare 2, an Over-Air Enabled System for General Purpose Data Channels”, published on Aug. 1, 1988 in the journal BBC Research and Development Report No 10, it is described an independent module (Conditional Access Module) in charge of receiving and decrypting the information necessary for the descrambling. This module manages the key dedicated to the system and delivers the control words necessary for the descrambling of the video broadcasts. In this type of module, once the control words are decrypted, they are transmitted uncoded to the unit in charge of the descrambling.
The problem then arises of how to make a receiver and a security module interdependent so that:                the given security module can be used only in the receiver for which the said security module was intended;        the information stream exchanged between the security module and the receiver is unique. This uniqueness prevents the public broadcasting of this stream from allowing other receivers to work without the security module provided for this purpose.        